Conflict Detection and Resolution Using Predicted Aircraft Trajectories

ABSTRACT

A method of detecting conflicts between aircraft passing through managed airspace, and to resolving the detected conflicts strategically. Air traffic control apparatus arranged to manage airspace through which aircraft are flying is provided that comprises processing means configured to receive aircraft intent data describing an aircraft&#39;s intended flight path, to launch a conflict detection procedure in which it computes a user-preferred trajectory for each of the aircraft based on the aircraft intent and determines whether any conflicts will arise, to launch a conflict resolution procedure in which it calculates revisions of the aircraft intent of the conflicted aircraft to remove the conflicts, and to transmit to the aircraft the revised aircraft intent data.

PRIORITY STATEMENT

This application claims the benefit of EP Patent Application No.12382206.6, filed on May 25, 2012 in the Spanish Patent Office, thedisclosure of which is incorporated herein by reference in its entirety.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to and incorporates herein by reference inits entirety, co-pending U.S. patent application Ser. No. 13/902,672,concurrently filed and entitled “Conflict Detection and Resolution UsingPredicted Aircraft Trajectories” which claims priority to EP PatentApplication No. 12382207.4, filed on May 25, 2012.

This application is related to and incorporates herein by reference inits entirety, co-pending U.S. patent application Ser. No. 13/902,697,concurrently filed and entitled “Conflict Detection and Resolution UsingPredicted Aircraft Trajectories” which claims priority to EP PatentApplication No. 12382208.2, filed on May 25, 2012.

This application is related to and incorporates herein by reference inits entirety, co-pending U.S. patent application Ser. No. 13/902,632,concurrently filed and entitled “Conflict Detection and Resolution UsingPredicted Aircraft Trajectories” which claims priority to EP PatentApplication No. 12382209.0, filed on May 25, 2012.

BACKGROUND INFORMATION

1. Field

This disclosure relates to automating the management of airspace. Inparticular, the present disclosure is concerned with detecting conflictsbetween aircraft passing through managed airspace, and to resolving thedetected conflicts strategically.

2. Background

Air traffic management is responsible for the safe passage of aircraftthrough an airspace. The aircraft may be manned or unmanned. To do this,a centralised, ground-based air traffic management facility mustcommunicate with aircraft flying through the airspace it manages. Thistwo-way communication may be done in a number of ways, including by oralcommunication such as by radio or by data communication through a datalink or the like.

The aircraft may determine their desired flight path through theairspace, for example using an airborne flight management system, andmay then communicate this to air traffic management. In modern times,air traffic management uses sophisticated computer systems to check thesubmitted flight paths do not result in aircraft trajectories that giverise to conflicts. Conflicts between aircraft arise when their intendedtrajectories would result in a separation falling below the minimumspecified. By trajectory, a four-dimensional description of theaircraft's path is meant such as a time-ordered sequence of aircraftstates, including position and altitude. Maintaining safe separations isa particularly demanding task, particularly in congested airspace suchas around airports where flight paths tend to converge.

In addition to detecting conflicts, air traffic management must have themeans to be able to resolve the conflicts and to communicate thenecessary changes in trajectories to the conflicted aircraft.

To date, most efforts aimed at air traffic management's ability todetect and resolve air traffic conflicts have focused on crossingtraffic patterns and have not dealt with the more challenging problem ofconverging traffic. This arises, for example, in arrivals management atTRACON (terminal radar control) facilities, where aircraft arrive frommany directions and must be sequenced for approach and landing at anairport. The efforts directed to converging traffic consider maximizingthe throughput of traffic on an airspace resource such as a sector or arunway as the main or sole objective when solving air traffic conflicts.Existing solutions also focus on planning the arrival sequence firstbefore detecting and resolving conflicts. The method then proceeds byextrapolating that sequence backwards to the earlier waypoints. However,such an approach only serves to propagate the delay backwards to allother aircraft.

Previous attempts at detecting and resolving conflicts suffer otherproblems. For example, previous attempts have analysed conflicts inisolation from each other, typically as isolated events between pairs ofaircraft. The detected conflicts are resolved in a sequential mannerwithout any consideration of the possibility of a “domino effect”feeding back delays.

Recent advances in predicting aircraft trajectories accurately are ofbenefit to air traffic management. In particular, work on expressingaircraft intent using formal languages provides a common platform forthe exchange of flight information and allows different interestedparties to perform trajectory calculations. For example, this aids thecommunication of planned trajectories between aircraft and air trafficmanagement.

EP patent application 07380259.7, published as EP-A-2,040,137, also inthe name of The Boeing Company, describes the concept of aircraft intentin more detail, and the disclosure of this application is incorporatedherein in its entirety by reference. In essence, aircraft intent is anexpression of the intent of how the aircraft is to be flown. Theaircraft intent is expressed using a set of parameters presented so asto allow equations of motion governing the aircraft's flight to besolved. The theory of formal languages may be used to implement thisformulation. An aircraft intent description language provides the set ofinstructions and the rules that govern the allowable combinations thatexpress the aircraft intent, and so allow a prediction of the aircrafttrajectory.

Flight intent may be provided as an input to an intent generationinfrastructure. The intent generation infrastructure may be airborne onan aircraft or it may be land-based such as an air traffic managementfacility. The intent generation infrastructure determines aircraftintent using the unambiguous instructions provided by the flight intentand other inputs to ensure a set of instructions is provided that willallow an unambiguous trajectory to be calculated. Other inputs mayinclude preferred operational strategies such as preferences withrespect to loads (both payload and fuel), how to react to meteorologicalconditions, preferences for minimising time of flight or cost of flight,maintenance costs, and environmental impact. In addition, other inputsmay include constraints on use of airspace to be traversed.

The aircraft intent output by the intent generation infrastructure maybe used as an input to a trajectory computation infrastructure. Thetrajectory computation infrastructure may be either located with or awayfrom the intent generation infrastructure. The trajectory computationinfrastructure may comprise a trajectory engine that calculates anunambiguous trajectory using the aircraft intent and other inputs thatare required to solve the equations of motion of the aircraft. The otherinputs may include data provided by an aircraft performance model and anEarth model. The aircraft performance model provides the values of theaircraft performance aspects required by the trajectory engine tointegrate the equations of motion. The Earth model provides informationrelating to environmental conditions, such as the state of theatmosphere, weather conditions, gravity and magnetic variation.

SUMMARY

Against this background, and from a first aspect, the present inventionresides in a computer-implemented method of managing airspace throughwhich a plurality of aircraft are flying.

The method comprises receiving, from the aircraft, user preferredaircraft intent data that unambiguously defines the user preferredtrajectory of each aircraft. The user-preferred aircraft intent data maybe a description of the aircraft's user-preferred trajectory expressedin a formal language or may be a full description of how the aircraft isto be operated expressed in a formal language that may be used tocalculate a corresponding unique trajectory.

The method further comprises calling an initial conflict detectionprocedure comprising calculating the corresponding user preferredtrajectories from the user preferred aircraft intent data, and comparingthe user preferred trajectories so as to identify one or more conflictsbetween trajectories. Aircraft predicted to fly the identifiedconflicting trajectories are also identified.

Then, the method further comprises calling an initial conflictresolution procedure comprising revising the user preferred aircraftintent data of one or more of the conflicted aircraft to produce revisedaircraft intent data that will unambiguously define a correspondingrevised trajectory.

In this way, the use of aircraft intent data may be conveniently usedfor air traffic management. Aircraft intent data may be shared betweenaircraft and an air traffic management facility. This allows the airtraffic management facility to check for conflicts and to suggestrevisions that remove these conflicts.

It is to be understood that an instance of aircraft intent dataunambiguously defines a trajectory, but the reverse is not true: anytrajectory may arise from more than one instance of aircraft intentdata. It has been realised that the air traffic management facilityshould operate to revise the user preferred aircraft intent datasubmitted by the aircraft, and to send the revised aircraft intent databack to the aircraft. It is aircraft intent data that is exchanged, notthe corresponding trajectories.

The method may comprise, after calling the initial conflict resolutionprocedure, performing a further conflict detection procedure. Thefurther conflict detection procedure may comprise calculating thecorresponding revised trajectories from the corresponding revisedaircraft intent data. The user-preferred trajectories and revisedtrajectories may be compared so as to identify one or more conflictsbetween trajectories. For example, the user-preferred trajectories ofthe aircraft not subject to revised aircraft intent data may be comparedwith revised trajectories from the aircraft subject to revised aircraftintent data. Aircraft predicted to fly the identified conflictingtrajectories are also identified. If conflicts are identified, themethod comprises either calling a further conflict resolution procedureor indicating that no further conflict resolution processing will takeplace. The further conflict resolution procedure may comprise revisingthe user preferred aircraft intent data and/or revised aircraft intentdata of one or more of the still-conflicted aircraft and calling thefurther conflict detection procedure. Thus a loop of further conflictresolution and detection procedures is created that loops until noconflicts remain. If no conflicts are identified, the method continuesto the step of transmitting revised aircraft intent data.

Both the air traffic management facility and the aircraft may use therevised aircraft intent data to generate the same, unique trajectory.This allows the air traffic management facility to check to ensure therevised aircraft intent data it generates successfully removes allconflicts, and also allows the aircraft to see and check the revisedtrajectory. For example, the aircraft may review the revised trajectoryand decide whether to accept or reject the revised aircraft intent data.

Thus, the method may further comprise receiving from an aircraft anindication that the revised aircraft intent data is rejected andperforming a rejection procedure. The rejection procedure may comprisecalling the further conflict resolution procedure in which the rejectedrevised aircraft intent data is further revised. Thus, the furtherconflict resolution and detection loop is re-entered such that either anew set of revised aircraft intent data is produced and sent to theaircraft. However, it may not be possible to derive a new conflict freeset of aircraft intent data, or there may not be enough computationaltime left, in which case the method may declare that no further conflictresolution processing will take place. In this case, the method mayfurther comprise advising the aircraft that rejected the revisedaircraft intent data that that aircraft must accept the rejected revisedaircraft intent data.

The method may comprise multiple rejection procedures. For example, anaircraft may have the opportunity to reject revised aircraft intent datamore than once, so triggering multiple rejection procedures. The numberof times an aircraft may reject revised aircraft intent data may belimited, for example to a predetermined number or according to availablerun time for the required rejection procedures. That is, if run time isgetting low, the rejection procedure may simply advise the aircraft thatit must accept the revised aircraft intent data.

Where revised aircraft intent data is rejected and a rejection procedurearises, the method may comprise performing the rejection procedure byforming locked and unlocked aircraft. The method may comprise lockingaircraft whose user preferred aircraft data was not revised and aircraftwho accepted revised aircraft intent data. The remaining aircraft withrejected revised aircraft intent data form the unlocked aircraft. Duringfurther conflict resolution procedures, aircraft intent data of lockedaircraft cannot be revised and aircraft intent data of unlocked aircraftcan be revised. This reduces the computation time of the rejectionprocedure as fewer aircraft have to be considered.

Optionally, the step of comparing the trajectories so as to identify oneor more conflicts between trajectories may comprise forming pairsbetween each of the conflicted aircraft. This may occur during theinitial conflict detection procedure and/or during the further conflictdetection procedure. For each pair of aircraft, the method may comprisecomparing the separation of the aircraft at discrete time points astheir trajectories progress. This may be performed for a pre-determinedtime window, e.g. the conflict detection procedure compares thetrajectories as they progress through a predetermined number of discretetime points.

Optionally, the separations may be checked against pre-definedseparation minimum, and conflicts may be detected based upon thedistance dropping below the minimum. The lateral and vertical distancebetween aircraft may be determined, and conflicts may be detected usingone or both of the lateral and vertical distances.

The conflict detection procedure may execute sequentially by determiningthe separation of all pairs of aircraft for each discrete time point,before moving onto the next discrete time point. The method may compriseapplying heuristics to identify pairs of aircraft that cannot come intoconflict at a particular discrete time point. The method may compriseusing the positions of aircraft in a pair to determine that they cannotcome into conflict for certain number of discrete time points. This maycomprise determining that the aircraft cannot come into conflict duringthe current discrete time point or may comprise determining the numberof discrete time points before they can come into conflict. For example,the positions may be used to calculate the time it would take for theaircraft to turn and approach each other at their maximum speeds andfinally for them to come into conflict. This may produce an answer of anumber of discrete time points for which that pair may be excluded fromdetermining if the aircraft come into conflict.

The above methods may be performed repeatedly, that is the method mayrepeat over and over to ensure the continued safe separation ofaircraft. Each iteration begins with an initial conflict detectionprocedure that may be performed repeatedly at predetermined intervals,periodically, whenever an aircraft enters the airspace, or when newuser-preferred aircraft intent data is received from an aircraft. Also,the initial conflict detection procedure may be performed repeatedly asa result of any combination of these triggers.

From a second aspect, the present disclosure resides in a method ofoperating an aircraft within a managed airspace. The method comprisesthe aircraft sending user preferred aircraft intent data thatunambiguously defines the user preferred trajectory of the aircraft toan air traffic management facility. The aircraft receives, from the airtraffic management facility, revised aircraft intent data thatunambiguously defines a corresponding revised trajectory of theaircraft. The aircraft calculates and displays the revised trajectory.The aircraft then follows the revised trajectory.

Optionally, after displaying the revised trajectory, the aircraft maysend to the air traffic management facility an indication that therevised aircraft intent data is rejected. Then, the aircraft may receivefrom the air traffic management facility further revised aircraft intentdata or an indication that the revised aircraft intent data must beaccepted. Finally, the aircraft follows either the further revisedaircraft intent data or the revised aircraft intent data, asappropriate.

The present disclosure also provides a method of operating an airspacecomprising a combination of any of the methods of managing an airspacedescribed above and any of the methods of operating an aircraftdescribed above.

The present disclosure also provides a computer apparatus programmed toimplement any of the methods described above, a computer programcomprising instructions that when executed on a computer, cause thecomputer to perform any of the methods described above, and a computerreadable medium having stored therein such a computer program.

The present disclosure also provides air traffic control apparatuscomprising a computer apparatus programmed to implement any of themethods of managing aircraft described above, and an aircraft configuredto implement any of the methods operating an aircraft described above.

From a third aspect, the present disclosure resides in an air trafficcontrol apparatus arranged to manage airspace through which a pluralityof aircraft are flying.

The air traffic control apparatus comprises communication meansconfigured for bi-directional communication with the aircraft, andprocessing means in operative communication with the communication meansso as to allow transfer of data to and from the aircraft.

The processing means comprises a traffic management component, atrajectory computation component and data storage means accessible toboth the traffic management component and the trajectory computationcomponent. The processing means is configured to receive, from theaircraft and via the communication means, user-preferred aircraft intentdata describing unambiguously the aircraft's user-preferred trajectoryof each aircraft and to store the user-preferred aircraft intent data inthe data storage means.

The traffic management component is configured to call an initialconflict detection procedure. This procedure comprises the trafficmanagement component calling the trajectory computation component tocalculate the corresponding user preferred trajectories from the userpreferred aircraft intent data, and the traffic management componentcomparing the user preferred trajectories so as to identify one or moreconflicts between trajectories and to identify conflicted aircraftpredicted to fly the identified conflicting trajectories.

The traffic management component is configured to call an initialconflict resolution procedure. This conflict resolution procedurecomprises the traffic management component revising the user preferredaircraft intent data of one or more of the conflicted aircraft toproduce revised aircraft intent data that will unambiguously define acorresponding revised trajectory. Then, the traffic management componentis configured to call a further conflict detection procedure comprisingthe traffic management component calling the trajectory computationcomponent to calculate the corresponding revised trajectories from therevised aircraft intent data, and the traffic management componentcomparing the user-preferred trajectories and revised trajectories so asto identify one or more conflicts between trajectories and to identifyconflicted aircraft predicted to fly the identified conflictingtrajectories.

The traffic management component is configured, if conflicts areidentified during the further conflict detection procedure, either tocall a further conflict resolution procedure or to indicate that nofurther conflict resolution processing will take place. The furtherconflict resolution procedure comprises revising the user preferredaircraft intent data and/or revised aircraft intent data of one or moreof the still-conflicted aircraft and to call the further conflictdetection procedure. If no conflicts are identified during the furtherconflict detection procedure, the method continues to a step oftransmitting revised aircraft intent data.

The processing means is configured, when no conflicts remain, to pass tothe communication means the revised aircraft intent data and thecommunication means is configured to perform the step of transmittingrevised aircraft intent data by transmitting the revised aircraft intentdata to the associated aircraft.

Optionally, the processing means is configured to receive, from theaircraft and via the communication means, an indication that the revisedaircraft intent data is rejected. Then, the processing means isconfigured to call a rejection procedure. The rejection procedurecomprises calling a further conflict resolution procedure in which thetraffic management component revises the rejected revised aircraftintent data. If further revised aircraft intent data is produced that isconflict free, the air traffic control apparatus sends the revisedaircraft intent data to the corresponding conflicted aircraft. If anindication that no further conflict resolution processing will takeplace is issued, the air traffic control apparatus advises the aircraftthat rejected the revised aircraft intent data that that aircraft mustaccept the rejected revised aircraft intent data.

Optionally, the processing means is configured to perform the rejectionprocedure by locking aircraft whose user preferred aircraft data was notrevised and aircraft who accepted revised aircraft intent data, theremaining aircraft being unlocked aircraft with rejected revisedaircraft intent data and, during further conflict resolution procedures,not to revise aircraft intent data of locked aircraft and to reviseaircraft intent data of one or more unlocked aircraft.

The traffic management component may be configured such that the step ofcomparing the trajectories so as to identify one or more conflictsbetween trajectories, during the initial conflict detection procedureand/or during the further conflict detection procedure, may compriseforming pairs between each of the conflicted aircraft and, for eachpair, comparing the separation of the aircraft on their trajectories atdiscrete time points as their trajectories progress. This may be done aspreviously described with respect to the first aspect of the presentdisclosure.

The air traffic control apparatus may be configured to repeat the loopof further conflict resolution and conflict detection procedures toensure safe separation of aircraft at all times. The cycles may repeatat predetermined intervals, periodically, whenever an aircraft entersthe airspace, or when new user-preferred aircraft intent data isreceived from an aircraft, or using any combination of these triggers.

The air traffic control apparatus may be provided together on a singlesite or may be distributed. Likewise, the processing means may beprovided together or may be distributed. The processing means may be oneor more computers. A single computer may provide the traffic managementcomponent and the trajectory computation component. Alternatively eitherthe flight management component or the trajectory computation componentor both may be distributed across more than a single computer, forexample by making use of a network of computers.

Optionally, the traffic management component and the trajectorycomputation component are modular such that the components may be easilyupdated or removed and replaced. For example, the components may bedesigned to receive inputs and to provide outputs according to apredefined standard. As a result of this arrangement, the air trafficcontrol apparatus may be used as a workshop test bench for testingimproved versions of the traffic management component and/or thetrajectory computation component.

From a fourth aspect, the present disclosure provides an aircraftcomprising airborne communication means for bidirectional communicationwith an air traffic control apparatus, and airborne processing means inoperative communication with the airborne communication means so as toallow transfer of data to and from the air traffic control apparatus.

The airborne processing means comprises a flight management component, atrajectory computation component and data storage means accessible toboth the flight management component and the airborne trajectorycomputation component. The flight management component is configured toprovide user-preferred aircraft intent data that describes unambiguouslythe aircraft's user-preferred trajectory. The airborne processing meansis configured to store in the data storage means the user-preferredaircraft intent data provided by the flight management component. Theairborne processing means is configured to transmit the user-preferredaircraft intent data to the air traffic control apparatus via theairborne communication means.

The airborne processing means is configured to receive from the airtraffic control apparatus, revised aircraft intent data thatunambiguously defines a corresponding revised trajectory of theaircraft, and to store in the data storage means the revised aircraftintent data. The airborne trajectory computation component is configuredto calculate the revised trajectory from the revised aircraft intentdata stored in the data storage means.

Optionally, the airborne processing means is configured to display arepresentation of the trajectory (to aid a pilot's comprehension), andto transmit an indication that the revised aircraft intent data isrejected to the air traffic control apparatus via the airbornecommunication means. The airborne processing means may be configured toreceive from the air traffic control apparatus, further revised aircraftintent data or an indication that the revised aircraft intent data mustbe accepted.

The aircraft is configured to follow the revised aircraft intent, or tofollow further revised aircraft intent data if appropriate.

As will be appreciated, some features of the air traffic controlapparatus are mirrored by corresponding features in the aircraft. Toallow corresponding features to be distinguished, the term airborne isused to identify those features provide on the aircraft.

Some or all of the aircraft flying in the airspace may be equipped withthe capabilities described above. Even where not all aircraft have thiscapability, the present disclosure may still be used with some or all ofthe aircraft that do have the capability.

The airborne processing means may be provided by one or more computers.A single computer may provide both the flight management component andthe trajectory computation component. As in common in aircraft systems,redundancy may be included by providing more than a single computer orsets of computers as the airborne processing means. The airborneprocessing means, or parts thereof, may be provided as part of anothersystem within the aircraft.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present disclosure may be more readily understood,preferred embodiments will now be described, by way of example only,with reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram showing aircraft flying within an airspacemanaged by an air traffic management facility;

FIG. 2 shows a framework illustrating the relationship between airtraffic management and an aircraft flying within the airspace it managesthat allows conflict detection and resolution;

FIG. 3 is a schematic representation of a negotiation process between anaircraft an air traffic management;

FIG. 4 is a flow chart representation of a method of detecting andresolving conflicts according to an embodiment of the present invention;

FIG. 5 is a flow chart representation of a method of detecting andresolving conflicts according to another embodiment of the presentinvention;

FIG. 6 is a flow chart representation of a system for detecting andresolving conflicts according to an embodiment of the present invention;

FIG. 7 is a flow chart representation of a conflict detection process;

FIGS. 8 a and 8 b show two examples of conflicting trajectories;

FIGS. 9 a and 9 b show two examples of how trajectories may be modifiedto resolve conflicts; and

FIG. 10 is a flow chart representation of a conflict resolution process.

DETAILED DESCRIPTION

The present disclosure provides methods and systems that enable aground-based airspace management system to de-conflict strategically thetrajectories of aircraft under its responsibility, regardless of whetherthe aircraft are manned or unmanned, in any traffic scenario includingconverging traffic patterns.

System Overview

FIG. 1 shows schematically an airspace 10 under the control of airtraffic management facility 12. In this example, air traffic management12 is located at an airport 14 and is responsible for aircraft 16arriving and departing from the airport 14, as well as those aircraft 16passing through the airspace 10.

Air traffic management 12 is provided with associated communicationmeans 18 to allow two-way communication with the aircraft 16 flyingthrough the airspace 10. The aircraft 16 are equipped with complementarycommunication equipment (not shown in FIG. 1) of any type well known inthe field of aerospace. For example, communication may be effected byradio or could be effected using a data link such as ADS-B.

Communication between air traffic management 12 and each of the aircraft16 is generally the same, and may be effected either in parallel orserially. A framework illustrating the relationship between air trafficmanagement 12 and one of the aircraft 16 will now be described in moredetail. It is to be understood that this framework is common to all theaircraft in the sense that it is the same for any aircraft 16 chosen tobe considered.

FIG. 2 shows schematically the airborne system 20, the ground-basedsystem 22, and the negotiation process 24 that occurs between theairborne system 20 and ground-based system 22. Airborne system 20 isprovided by the aircraft 16, and the ground-based system 22 is providedby air traffic management 12. The negotiation process 24 requires acommunication system 26 that is distributed between the aircraft 16 andair traffic management 12, namely a transmitter/receiver provided on theaircraft 16 and the communication means 18 provided at the air trafficmanagement facility 12.

In the example of FIG. 2, the communication system 26 is used toexchange aircraft intent data 28 between the airborne automation system20 and the ground-based automation system 22. The aircraft intent data28 may be provided by the airborne automation system 20 or by theground-based automation system 22. The aircraft intent data 28 providedby the airborne automation system 20 will correspond to theuser-preferred trajectory of the aircraft 16, whereas the aircraftintent data 28 provided by the ground-based automation system 22 willcorrespond to a revised trajectory determined by air traffic management12.

The airborne automation system 20 comprises flight management logic 30and trajectory computation infrastructure 32. Both these components arecomputer-implemented, preferably as separate computer systems. Forexample, the flight management logic 30 may be part of a flight computerof the aircraft 16.

The flight management logic 30 is responsible for following andsupervising the negotiation process 24 from the aircraft's point ofview. The flight management logic 30 is also responsible for definingthe user-preferred aircraft intent data 28 and agreeing the revisedaircraft intent 28 with the ground-based automation system 22.

The trajectory computation infrastructure 32 is responsible forcomputing the trajectory resulting from a given flight intent 28. Forexample, it may calculate the trajectory arising from a user-preferredaircraft intent for presentation to a pilot for approval before thecorresponding user-preferred aircraft intent data 28 is provided to theground-based automation system 22. Additionally, the trajectorycomputation infrastructure 32 may generate and display a trajectorycorresponding to revised aircraft intent data 28 provided by theground-based automation system 22 such that the pilot may approve therevised trajectory.

The ground-based automation system 22 comprises traffic management logic34 and trajectory computation infrastructure 36. Both these componentsare computer-implemented, preferably as separate computer systems.Although the trajectory computation infrastructure 36 performs a similarfunction to the trajectory computation infrastructure 32 of the airborneautomation system 20, it need not be the same and may be implementeddifferently.

The traffic management logic 34 is responsible for following andsupervising the negotiation process 24. The traffic management logic 34is also responsible for revising aircraft intents where conflicts arise.To enable revision of the aircraft intents, the traffic management logic34 has at its disposal algorithms relating to a look ahead process thatgoverns when to run a conflict detection process, to conflict detectionand to conflict resolution. In this example, the traffic managementlogic 34 is modular in its nature such that any of the algorithms may bevaried or entirely replaced without affecting the other algorithms. Thismodularity also makes the traffic management logic 34 ideal as a testbed for developing improved algorithms in that revised versions of thealgorithm may be readily swapped in and out of the traffic managementlogic 34.

The trajectory computation infrastructure 36 is responsible forgenerating trajectories corresponding to aircraft intents at theground-based automation system 22. The aircraft intent may be theuser-preferred aircraft intent 28 received from the airborne automationsystem 20 or the revised aircraft intent 28 determined by the trafficmanagement logic 34.

The negotiation process 24 defines the type of information to be sharedbetween the airborne automation system 20 and ground-based automationsystem 22. The negotiation process 24 also defines who is to startcommunication according to what events, and the sequence of decisions tobe followed in order to agree upon a revised aircraft intent 28. FIG. 3shows the steps of the negotiation process 24, and will now be describedin more detail.

In this example, the negotiation process 24 starts on-board the aircraft16 with the definition of the aircraft's intent that corresponds to auser-preferred trajectory. This is shown in FIG. 3 at 40. The aircraft16 establishes contact with air traffic management 12 and transmits theuser-preferred trajectory information 42 expressed as the user-preferredaircraft intent data 28 a to air traffic management 12.

Once the user-preferred aircraft intent data 28 a has been received, theaircraft 16 and air traffic management 12 engage in a one-to-onenegotiation process. During the negotiation process 24, theuser-preferred aircraft intent data 28 a submitted by the aircraft 16 isused by the trajectory computation infrastructure 36 to produce thecorresponding trajectory. This user-preferred trajectory is analyzed bythe traffic management logic 34 in order to detect potential conflictswith other aircraft trajectories.

When conflicts are detected, the airborne automation system 20 and theground-based automation system 22 will follow the predeterminednegotiation protocol required by the negotiation process 24 to agree ontrajectory modifications to remove the conflict. The negotiation process24 includes exchange of trajectory information as the aircraft intentdata 28 and, as this is a common characteristic to all possiblenegotiation protocols, it advantageously allows the protocols to beinterchangeable.

Once the user-preferred aircraft intent data 28 a has been received byair traffic management 12 as shown at 44 in FIG. 3, the negotiationprocess 24 continues with a look-ahead process at 46. The look-aheadprocess 46 operates to determine when to launch a conflict detectionprocess 110 and which aircraft (and their trajectories) have to beincluded in that process. Different look-ahead processes 46 may beimplemented as long as pre-established interfaces are maintained.

The look-ahead process 46 may run the conflict detection process 110periodically. The rate of repetition may be varied, for exampleaccording to the volume of air traffic. In addition or as analternative, the conflict detection process 110 may be invoked whenevera new aircraft enters the managed airspace. Further details are givenbelow.

Once the look-ahead process 46 decides which aircraft 16 are going to beincluded in the conflict detection process 110, the conflict detectionprocess 110 is launched. Here, as well, different conflict detectionprocesses 110 may be implemented as long as the pre-establishedinterfaces are maintained. In summary, the conflict detection process110 computes the user-preferred trajectories corresponding to theuser-preferred aircraft intent data 28 a received, and analyses thetrajectories computed to identify potential conflicts. When anyconflicts are identified by the conflict detection process 110, theconflict resolution process 120 is launched.

The conflict resolution process 120 performs calculations to revise theuser-preferred aircraft intent data 28 a to generate revise aircraftintent data 28 b. The revised intents result in corresponding revisionsto the user-preferred trajectories in order to remove the identifiedconflicts. Different conflict detection processes 110 may be implementedas long as the pre-established interfaces are maintained.

As will be explained below, the conflict resolution process 120 callsthe conflict detection process 110 to analyse the revised trajectoriesresulting from the revised aircraft intent data 28 b it proposes toensure that no conflicts remain and that no new conflicts are generated.Once it is confirmed that no conflicts arise, the revised aircraftintent data 28 b are transmitted to the affected aircraft 16 by airtraffic management 12, as shown at 52 in FIG. 3.

The revised aircraft intent data 28 b are received by the aircraft 16under the current consideration, as shown at 54. The aircraft 16 maygenerate a corresponding revised trajectory. In some embodiments, theaircraft 16 is obliged to follow the revised trajectory defined by therevised aircraft intent data 28 b. In other embodiments, including theembodiment currently being described, the aircraft 16 is given theoption of rejecting the revised aircraft intent data 28 b. In this casea further round of negotiation is required or, if time does not allow,the aircraft 16 may be commanded to accept the revised trajectory by theground-based automation system 22. The further round of negotiation maysee a new set of revised aircraft intent data 28 b sent to the aircraft16 for review of the corresponding new revised trajectory. If improvedaircraft intent data 28 b cannot be found, or if computation time forthe negotiation process runs out, the ground-based automation system 22may command the aircraft 16 to follow the original aircraft intent data28 b. In any event, once revised aircraft intent data 28 b is acceptedand the corresponding trajectory is executed by the aircraft 16 as shownat 56. As will be appreciated, the conflict detection and resolutionprocess is a dynamic process, and so further changes may be imposed onthe trajectory as it is executed by the aircraft 16.

Conflict Detection and Resolution Overview

Methods of detecting and resolving conflicts in predicted aircrafttrajectories are now described. These methods ensure that the resolvedtrajectories do not result in further conflicts downstream, henceavoiding a “domino effect” of conflicting trajectories propagatingbackwards through the chain of aircraft.

The overall conflict detection and resolution process may be envisagedas a two-stage process of firstly detecting conflicts and secondlyresolving the conflicts. This is illustrated in FIG. 4 by the dashedboxes 110 and 120. Generally, an initial stage of obtaininguser-preferred trajectories of aircraft 16 is performed, as shown bydashed box 100 in FIG. 4. Also, a final stage of advising aircraft 16 ofrevised aircraft intent data 28 b is generally performed, as indicatedby dashed box 130 in FIG. 4. A more detailed description of the fullerfour-stage method of FIG. 4 will now be provided.

The method of FIG. 4 may be practised by a ground-based automationsystem 22 hosted at an air traffic management facility 12, for exampleusing a network of computers located at the facility 12, as describedabove. Air traffic management 12 will assume responsibility for the safepassage of aircraft through the airspace 10 that it manages. The methodstarts at 101 where user-preferred trajectories of the aircraft 16flying through the managed airspace 10 are obtained. This may be done inseveral different ways. For example, a description of the user-preferredtrajectories may be provided. Alternatively, the trajectories may becalculated and hence predicted as part of the method. A description ofan aircraft's user-preferred intent may be provided, for exampleexpressed using a formal language, as shown at 28 in FIG. 2. Air trafficmanagement 12 may then use this user-preferred aircraft intent data 28to calculate a user-preferred trajectory for the aircraft 16.

With the trajectory prediction process 100 complete, the method moves tothe conflict detection process 110. At step 111, aircraft trajectoriesare compared and conflicts identified. This process is described in moredetail below. At 112, the aircraft 16 predicted to fly conflictingtrajectories are identified and these aircraft are nominally placed intoa set of conflicted aircraft at step 113.

The method then progresses to the conflict resolution process 120. Atstep 121, the set of aircraft formed at step 113 is used. Theuser-preferred aircraft intent data 28 a of aircraft identified withinthe set of conflicted aircraft are adjusted and the correspondingtrajectories calculated to identify one or more instances where allconflicts are resolved.

Once the conflicts are resolved, the method may progress to process 130where conflicted aircraft 16 are advised of their revised aircraftintent data 28 b. This may involve it may involve sending a descriptionof the associated aircraft intent such that the aircraft 16 may thencalculate the corresponding trajectory or transmitting a description ofthe new trajectory to the aircraft 16. The former example was describedabove. As a description of aircraft intent is by definition a set ofinstructions that unambiguously define a trajectory, it is assured thatthe aircraft 16 will generate the intended trajectory.

As will be appreciated, the above method will be performed repeatedly byair traffic management 12. This accounts for variable conditions thatmay otherwise affect the calculated trajectories. For example,unexpected winds may give rise to conflicts that were not previouslypredicted. Repetition of the method may also be used to check thataircraft 16 are indeed following the user-preferred and revisedtrajectories and that the airspace remains free of predicted conflicts.Although the rate of repetition may be varied, as an example the methodmay be repeated at set intervals of every thirty seconds. In addition oras an alternative, the method may be invoked whenever a new aircraft 16enters the managed airspace 10. As well as including all aircraft 16within the managed airspace 10, the method may also consider aircraft 16approaching the airspace 10.

FIG. 5 shows another method of managing an airspace 10, includingdetecting and resolving trajectories of aircraft 16, according to anembodiment of the present invention. According to the embodiment of FIG.2, the method is integrated in a ground-based automation system 22 andworks as follows.

At 102, the traffic management logic 34 of the ground-based automationsystem 22 receives a description of the user-preferred trajectories ofthe aircraft within its area of responsibility. The trajectories aredescribed by the user-preferred aircraft intent data 28 a expressedusing an aircraft intent description language.

At 103, the traffic management logic 34 sends the user-preferredaircraft intent data 28 a to the trajectory computation infrastructure36 that processes those data and predicts the correspondinguser-preferred trajectories.

At 114, possible conflicts are identified, i.e. instances where theseparation between user-preferred trajectories are in violation ofestablished minimum distances between the aircraft 16.

At 115, the detected conflicts are grouped into conflict dependentnetworks. Each network includes all aircraft 16 in conflict with atleast one other aircraft 16 within the network. For example, if aircraftA1 conflicts with aircraft A2, and aircraft A2 conflicts with aircraftA3 and A4 and aircraft A4 conflicts with aircraft A5, a conflictdependent network is formed containing aircraft A1, A2, A3, A4 and A5.All aircraft 16 within the network have conflict dependencies on thetrajectories of all the other aircraft 16 in the network, eitherdirectly or indirectly. A consequence of these types of networks is thatany particular aircraft 16 can be a member of only one conflictdependent network.

At 120, the conflicts are resolved “network-wise”, i.e. consideringsimultaneously all conflicts in a conflict dependent network. In thisway, the implications of the resolution actions on other conflictswithin the network are taken into account from the outset. Theresolution actions are the actions needed to be taken by an aircraft 16to avoid the conflict. These actions are designed as amendments to theuser-preferred aircraft intent data 28 a that produce revisedtrajectories.

As indicated at 122, the resolution actions for the conflicting aircraftwithin a conflict dependent network are selected from a set of jointcandidate resolution strategies (JCRS). The joint candidate resolutionstrategies are derived from a set of predefined joint candidateresolution patterns (JCRP). The selection is carried out so that theselected joint candidate resolution strategy belongs to a set ofPareto-optimal joint candidate resolution strategies. This set of jointcandidate resolution strategies that solve each conflict dependentnetwork are gathered together at step 123. Pareto optimality in thiscontext may be defined in different ways. For example, it may relate tothe changes in flight times or it may relate to a joint cost functioncapturing the additional operating costs, resulting from the resolutionactions as applied across all the aircraft 16 in the conflict dependentnetwork. Thus, the resolution actions in the selected joint candidateresolution strategy are such that the aircraft 16 belonging to the sameconflict dependent network share the consequences of the trajectorymodifications required to resolve the conflicts. For example, thestrategy that sees more, shorter delays spread across more aircraft 16may be preferred to a strategy that sees fewer, larger delays applied toonly a few aircraft 16. At step 124, the optimum joint candidateresolution strategy is selected for each conflict dependent network.

Once the joint candidate resolution strategy has been selected, theaircraft 16 whose trajectories have been amended are identified and therevised aircraft intent data 28 b are communicated to the affectedaircraft 16, as indicated at 132.

In this way, it is possible to solve the problem of resolving airtraffic conflicts strategically in a trajectory-based operationalenvironment by sharing consequences of changes resulting from theresolution of the conflicts among all aircraft involved.

FIG. 6 shows a further embodiment of a ground-based automation system300, that may be used to implement the method of FIG. 4 or FIG. 5. Theground-based automation system 300 comprises three sub-systems, namely atrajectory prediction module 302, a conflict detection module 304 and aconflict resolution module 306.

The ground-based automation system 300 receives as an input adescription of the trajectories of the aircraft expressed asuser-preferred aircraft intent data 28 a using an aircraft intentdescription language (AIDL), as indicated at 301.

The trajectory prediction module 302 calculates the user-preferredtrajectories and provides them as output 303. The user-preferredtrajectories 303 are taken as an input by the conflict detection module304.

The conflict detection module 304 uses the user-preferred trajectoriesto detect conflicts and to group the conflicts into conflict dependentnetworks, as has been described above. The conflict detection module 304provides the conflict dependent networks as an output 305 that isprovided to the conflict resolution module 306.

The conflict resolution module 306 operates on the conflict dependentnetworks to produce joint candidate resolution strategies for eachconflict dependent network, and outputs a joint candidate resolutionstrategy at 307. This joint candidate resolution strategy is used todetermine the data to be sent to affected aircraft by a communicationsystem 308. Although the communication system 308 is shown as beingseparate to the ground-based automation system 300, it may be a part ofthe ground-based automation system 300. For example, the modules 302,304 and 306 and, optionally, the communication system 308 may beprovided as a computer system. The computer system may comprise a singleserver, a plurality of servers and may be provided at a single locationor as part of a distributed network.

As noted above, the two key processes in the method are the conflictdetection process 110 and the conflict resolution process 120. Each ofthese processes will now be described in more detail.

Conflict Detection

FIG. 7 shows the steps involved in a preferred form of the conflictdetection process 110. FIG. 7 shows the process 110 starting at 402. Atstep 404, data is collected. Specifically, a conflict detection (CD)list of aircraft 405 is compiled. The aircraft list 405 to be consideredby the conflict detection process is the list of aircraft 405 known atthe time when the conflict detection and resolution processes arelaunched.

Each aircraft 16 in the aircraft list 405 must have associated certainpieces of information that are required to carry out the conflictdetection process 110. These pieces of information are referred to asconflict detection attributes, and are initially provided together withthe aircraft list 405. The conflict resolution process 120 may in turnalter the conflict detection attributes when subsequently calling theconflict detection process 110 in order to verify whether the revisedaircraft intent data 28 b and the corresponding revised trajectories areindeed conflict free. The main conflict detection attributes aredescribed below.

Type: each aircraft 16 in the list 405 is marked as either available orunavailable, or “unlocked” and “locked” as they will be referred tohereinafter. An aircraft 16 has a preferred trajectory that it wouldlike to fly. That trajectory is expressed as the aircraft intent or, inother words, how the aircraft would like to fly that trajectory. If thatintention to fly can still be changed, this means the aircraft 16 andair traffic management 12 have not yet agreed to it, in which case theaircraft is available or unlocked. If it cannot be changed, the aircraft16 is unavailable or locked.

Initial conditions: the available aircraft 16 have associated anestimated time and aircraft state at sector entry (i.e. at the time ofentering the managed airspace 10). These data represent the predictedinitial conditions of the aircraft 16 at sector entry and theseconditions are the starting point for the predictions and search forconflicts.

Current aircraft intent: the current aircraft intent of an unlockedaircraft may be that aircraft's user-preferred aircraft intent data 28a, or revised aircraft intent data 28 b resulting from a previousconflict detection and resolution process.

At 406, the timeline of the current conflict detection and resolutionprocess is discretized.

Next, at 408, the conflict detection process 110 calls a trajectorypredictor (TP) of the trajectory computation infrastructure 36 topredict the trajectories within its sector for all the aircraft 16 inthe aircraft list 405 from the current simulation time forward. Theinputs to the trajectory computation process are the initial conditionsand the current aircraft intent 28 provided as the aircraft's conflictdetection attributes. This provides the aircraft state at eachprediction time step for all aircraft 16, as indicated at 409.

Once the trajectory predictions are available, the conflict detectionprocess 110 starts calculating the evolution of the inter-aircraftdistances for all possible aircraft pairs along the prediction timeline.In this embodiment, the term inter-aircraft distance refers to theshortest distance over the Earth's surface between the groundprojections of the position of two aircraft 16. Inter-aircraft distanceis used because it is assumed that aircraft 16 must maintain horizontalseparation at all times and that, consequently, the separation minimaapplicable are expressed in terms of inter-aircraft distance, e.g. radarseparation or wake vortex separation. Thus, a conflict occurs when thepredicted inter-aircraft distance between two aircraft 16 falls belowthe applicable minimum during a certain time interval. The conflictdetection process 110 has access to a database containing the applicableminima, which are inter-aircraft distance values that must not beviolated. These minima may depend on the aircraft type, and the relativeposition of the aircraft 16 (e.g. wake vortex separation may prevailbetween aircraft 16 following the same track, but not between aircraft16 on converging tracks). During this process, regard may be paid to thevertical separation of aircraft 16, e.g. to allow reduced horizontalseparation where the vertical separation is sufficient to allow this.

The conflict detection process 110 starts at step 410 where theinter-aircraft distances are calculated for the initial conditions, i.e.the origin of the timeline. Next, at step 412, all possible pairs ofaircraft 16 are formed as shown at 413, and heuristics are applied toeach pair of aircraft 16. At each time step, the conflict detectionprocess 110 applies some heuristics before calculating theinter-aircraft distances, in order to skip aircraft pairs that, giventhe prior evolution of their inter-aircraft distance and their relativepositions, cannot possibly enter into a conflict during the current timestep. In addition, other heuristics will be in place to accelerate thecalculation of the inter-aircraft distances and the comparison with theapplicable minima.

Once the heuristics have been applied, the remaining aircraft pairs havetheir inter-aircraft distances calculated at 414. These inter-aircraftdistances are checked against the applicable separation minima at 416.At 418, the list of conflicts is updated with the newly identifiedconflicts. This step includes creating the new conflicts in the list andupdating associated attributes, as shown at 419.

Once step 418 is complete, the conflict detection process 110 canproceed to the next time step, as shown at 420. A check is made at step422 to ensure that the next time step is not outside the predictionwindow as indicated at 423 (i.e. the conflict detection process willlook forward over a certain time window, and the time steps should moveforward to cover the entire window, but should not go beyond thewindow). Provided another time step is required, the conflict detectionprocess 110 loops back to step 412 where heuristics are applied for thenext time step.

In this way, the conflict detection process 110 proceeds along theprediction time line, from the start to the end of the predictionwindow, calculating the inter-aircraft distance between all possibleaircraft pairs at each time step. The conflict detection process 110 isable to identify all conflicts between the aircraft 16 in the aircraftlist 405 between the start and end of the prediction timeline. Theconflict detection process 110 compiles the identified conflicts into aconflict list, where each conflict is associated with the followingpieces of information, denoted as conflict attributes.

Conflicting aircraft pair: identifiers of the two conflicting aircraft16, together with their conflict detection attributes.

Conflict type: an identifier associated to the type of conflict. In thisparticular embodiment, only two types of conflicts can occur. The firsttype, catching-up conflicts, is shown in FIG. 8 a where the loss ofseparation occurs between aircraft 16 flying along the same track, i.e.their separation dactual falls below the minimum separation alloweddmin. The second type, merging conflicts, is shown in FIG. 8 b where theloss of separation takes place between two aircraft 16 on convergingtracks as they approach the merging point, i.e. their separation dactualfalls below the minimum separation allowed dmin.

Conflict interval: the time interval, in the prediction timeline, duringwhich the inter-aircraft distance is below the applicable minimum.

Conflict duration: the length, in time steps, of conflict interval, i.e.the number of times steps during which the inter-aircraft distance isbelow the applicable minimum.

Conflict intensity: this attribute is a value between 0 and 10 thatprovides a measure of the severity of the conflict (with 0 being thelowest level of severity and 10 the highest). The conflict intensity isa function of the minimum predicted inter-aircraft distance during theconflict and is calculated taking into account the proportion of theapplicable minimum violated by that minimum distance. For example, aminimum predicted separation of 2 miles will result in a conflictintensity of 4.0 when the applicable minimum is 5 miles, and 6.7 whenthe applicable minimum is 3 miles.

Aircraft intent instructions associated with the conflict: the conflictdetection process 110 associates the set of aircraft intent instructionsthat are active for each of the two conflicting aircraft during theconflict interval.

Subsequently, at 424, the identified conflicts are grouped into conflictdependent networks according to an equivalence relation (called theconflict dependency relation) that is defined over the set ofconflicting aircraft 16. This equivalence relation is in turn based onanother relation defined over the set of conflicting aircraft 16, namelythe conflict relation (‘A belongs to the same conflicting pair as B’),which establishes that an aircraft A1 is related to an aircraft A2 ifthey are in conflict with each other (or they are the same aircraft).The conflict relation is not an equivalence relation, as it does nothave the transitive property (if A1 is in conflict with A2 and A2 is inconflict with A3, A1 is not necessarily in conflict with A3). Theconflict dependency relation is based on the conflict relation asfollows: two aircraft 16 are considered related (equivalent) accordingto the conflict dependency relation if it is possible to connect them bymeans of a succession of conflict relations. It is easy to check thatthis relation fulfils the three properties of equivalence: reflexive,symmetric and transitive.

As an example, let us consider an aircraft A1 anticipated to enter inconflict with two different aircraft, A2 and A3, during a certainsegment of its trajectory. In addition, let us assume that A3 will alsocome in conflict with another aircraft, A4. As a result, the followingconflicts (conflict relations) will take place: A1-A2, A1-A3 and A3-A4.From these conflict relations it can immediately be seen that A1 isequivalent to A2 and to A3 and that A3 is equivalent to A4. In addition,by the transitive property A2 is equivalent to A3 (applying the conflictdependency relation: A2 is in conflict with A1, which is in conflictwith A3), A1 is equivalent to A4 (applying the conflict dependencyrelation: A1 is in conflict with A3, which is in conflict with A4) andA2 is equivalent to A4 (applying the conflict dependency relation: A2 isin conflict with A1, which is in conflict with A3, which is in conflictwith A4). Thus, the four aircraft 16 belong to the same equivalenceclass. The elements of an equivalence class are equivalent, under theequivalence relation, to all the others elements of the same equivalenceclass. Any two different equivalence classes in a non-empty set aredisjoint and the union over all of the equivalence classes is the givenset.

In the present context, the equivalence classes defined by the conflictdependency equivalence relation are the conflict dependency networksmentioned previously. It will now be understood that the aircraft 16belonging to each conflict dependent network are interconnected throughconflict dependency relations. Considering the properties of equivalencerelations, conflict dependent networks are disjoint, i.e. two aircraft16 cannot belong to two conflict dependent networks simultaneously. Inthe example above, A1, A2, A3 and A4 form a conflict dependent network.

Considering the above, the conflict detection process 110 first groupsthe conflicting aircraft 16 into conflict dependent networks at 424(using the information in the conflict list), and then groups theconflicts between the aircraft 16 in each conflict dependent networkinto a conflict sub-list. The conflict list contains as many sub-listsas there are conflict dependent networks. Analogously to the conflictdependent networks, conflict sub-lists are disjoint and their union isthe conflict list. Finally, the conflict detection process 110 ordersthe conflicts in each sub-list chronologically (earlier conflicts first)based on the first time step at which the applicable minimum is firstviolated (the start of the conflict interval).

Conflict Resolution

Completion of the conflict detection process 110 causes the conflictresolution process 120 to be called. The conflict detection process 110provides the conflict resolution process 120 with the conflict listorganized as a set of conflict sub-lists, each corresponding to aconflict dependent network.

The conflict resolution process 120 modifies the current aircraft intentdata 28 of at least some of the conflicting aircraft 16 so that theresulting trajectories are predicted to remain conflict-free and asefficient as possible. The conflict resolution process 120 only altersthe aircraft intent data 28 of the unlocked aircraft 16 in the conflictlist. Thus, it is assumed that there can be no conflicts involving onlylocked aircraft (these conflicts would have been resolved in a previousiteration of the conflict detection and resolution processes).

The conflict resolution process 120, for example in the case of arrivalmanagement, measures efficiency on the basis of predicted RunwayThreshold Crossing Time (tRT) for the aircraft 16. In particular, theobjective of the conflict resolution process 120 is to alter theaircraft intent data 28 in such a way that the resulting estimated valueof tRT deviates the least possible from the value that would be obtainedwith the user-preferred aircraft intent data 28 a.

The conflict resolution process 120 operates in a network-wise manner,attempting to get the aircraft 16 belonging to the same conflictdependent network to share equally the delays incurred in resolving theconflicts in which they are involved.

Let us assume that the conflict detection aircraft list 405 contains naircraft grouped into m disjoint conflict dependent networks. Let us nowconsider the conflict dependency network CDN_(j)={A₁ ^(j), . . . , A_(i)^(j), . . . , A_(n) _(j) ^(j)}, with iε{1, . . . , n_(j)}, jε{1, . . . ,m_(j)} and

${\sum\limits_{j}n_{j}} = {n.}$

All the conflicts in which an aircraft A_(j) ^(i)εCDN_(j) is involvedare contained in the conflict sub-list associated to CDNj, denoted asSLj. A candidate resolution strategy (CRS) for an aircraft A_(i)^(j)εCDN is an instance of aircraft intent that, if implemented by A_(i)^(j) could potentially result in a conflict-free trajectory for theaircraft 16. In principle, any feasible aircraft intent for A_(i) ^(j)that is operationally meaningful in the scenario considered could beconsidered a candidate resolution strategy for that aircraft 16(including its preferred aircraft intent) since a conflict may beresolved as a result of actions. Candidate resolution strategies arederived from a set of pre-defined candidate resolution patterns (CRPs),which capture the allowable degrees of freedom that the aircraft 16 haveat its disposal to resolve conflicts in the scenario considered.Different CRPs target different conflict problems, for example someassist in an aircraft catching up and coming into conflict with anearlier aircraft and some assist in an aircraft falling behind intoconflict with a following aircraft. Selection of appropriate CRPs may bemade, as is described in more detail below.

A joint candidate resolution strategy (JCRS) for CDNj is a setcomprising of nj candidate resolution strategies, each assigned to oneof the aircraft in CDN_(j): JCRS_(j)={CRS₁ ^(j), . . . , CRS_(i) ^(j), .. . , CRS_(n) _(j) ^(j)}, with JCRSj denoting a JCRS for CDNj andCRS_(i) ^(j) denoting a candidate resolution strategy for the aircraftA_(i) ^(j)εCDN_(j). A conflict-free JCRSj is a joint candidateresolution strategy for CDNj that is predicted to result in no conflictsinvolving the aircraft 16 in CDNj, i.e. SLj would become empty as aresult of implementing a conflict-free JCRSj. To check whether a JCRSjis conflict-free, the conflict resolution process 120 must call theconflict detection process 110.

The objective of the conflict resolution process 120 is to design aconflict-free JCRSj that distributes the cost of resolving the conflictsin SLj among the aircraft belonging to CDNj in the most equitable waypossible.

It is assumed that the cost incurred by an aircraft A_(i) ^(j) as aresult of implementing a strategy CRSS_(i) ^(j) is measured by thedeviation that CRSS_(i) ^(j) causes from the aircraft operator'sobjectives (for the whole trajectory or a segment). These objectives arecaptured by the timeline corresponding to the trajectory that resultsfrom flying according to the user-preferred aircraft intent data 28 aand that is denoted as t_(RT) ^(pref). Thus, the cost of a candidateresolution strategy CRS_(i) ^(j) for A_(i) ^(j) is defined as follows:

c(CRS_(i) ^(j))=|t _(RI)(CRS_(i) ^(j))−t _(RI) ^(pref)|  (1)

where c(CRS_(i) ^(j)) is the cost of CRS_(i) ^(j), t_(RT)(CRS_(i) ^(j))is the arrival time for aircraft A_(i) ^(j) that is expected to resultfrom flying CRS_(i) ^(j).

As it stems from equation (1), the cost of delay and early arrival areconsidered to be the same. Thus, it is implicitly assumed that it is ascostly for the airline to arrive early as to arrive late. The costfunction could be adjusted to encode a higher cost of delay versus earlyarrival. For example, removing the absolute value from |t_(RT)(CRS_(i)^(j))−t_(RT) ^(pref)| in (1) would result in early arrivals having anegative cost, which would capture a situation where the airlineconsiders rewarding an early arrival.

Considering the above, the cost of a CRS measures the difference betweenthe arrival time that would result from flying the candidate resolutionstrategy and those that would result from flying the user-preferredaircraft intent 28 a, with the latter being the values preferred by theoperator. Thus, the cost of implementing the preferred aircraft intentas a CRS is zero, as it would result in no deviation from theuser-preferred arrival time.

In light of the above, the resolution of the conflicts in certainconflict sub-lists is cast as a constrained multi-objective optimizationproblem over the corresponding conflict dependent network. The problemis stated as follows:

minimise c(JCRS_(j))=(c(CRS_(i) ^(j)), . . . ,c(CRS_(i) ^(j)), . . .,c(CRS_(n) _(j) _(j)))

subject to JCRS_(i) εD _(j) ,D⊂X _(j)  (2)

where c(JCRS_(j)) is a vector function, with image in R^(n) ^(j) ,defined over the set Xj, which is the set of all possible jointcandidate resolution strategies for SLj. A vector c(JCRS_(j)) includesthe costs derived from each of the candidate resolution strategiescontained in JCRSj, a joint candidate resolution strategy for theaircraft 16 in CDNj. Dj denotes the set of conflict-free joint candidateresolution strategies for those aircraft 16. The solution to the problemin (2) would be one (or more) JCRS_(j)εD_(j) that simultaneouslyminimize, in some appropriate sense, the resolution costs as defined in(1) for all the aircraft 16 in the network.

It is not possible to define a single global optimum for a problem suchas the one in (2). Instead, as it is commonly done in multi-objectiveoptimization problems, we will assume that the solution consists of aset of acceptable trade-offs among the costs incurred by the aircraft16. The set of trade-offs considered is the Pareto set, which comprisesof all the Pareto-optimal solutions. A Pareto-optimal solution of (2) isa conflict-free JCRSj that is optimal in the sense that no otherconflict-free JCRSj can reduce the cost for an aircraft 16 in CDNjwithout increasing the cost for at least one other aircraft 16. Tocharacterize mathematically the Pareto set, it is necessary to extendthe relational operators =, ≦ and < to the set Z_(j)=Im(c(D_(j))), whichis the image of Dj on R^(n) ^(j) , i.e. Z_(j) ⊂R^(n) ^(j) . Thus,c(JCRS_(j))εZ_(j) ⊂R^(n) ^(j) . For any two vectors u, v εZ_(j), thefollowing relationships are defined:

u=v if ∀iε{1, . . . ,n _(j) }:u _(i) =v _(i)

u≦v if ∀iε{1, . . . ,n _(j) }: u _(i) ≦v _(i)

u<v if u≦v and u≠v  (3)

Considering the definitions in (3), a conflict-free joint candidateresolution strategy JCRS*_(j) is said to be a Pareto-optimal solution tothe problem (2) if there is no JCRS_(j)εD such that

c(JCRS_(j))<c(JCRS*_(j))  (4)

The individual candidate resolution strategies that make up aPareto-optimal solution are denoted as CRS₁ ^(j)*, . . . , CRS_(i)^(j)*, . . . , CRS_(n) _(j) ^(j)*. Considering the individual costs inc(JCRS_(j)*), given by c₁(JCRS_(i)*)=c(CRS₁ ^(j)*), . . . ,c_(i)(JCRS_(i)*)=c(CRS_(i) ^(j)*), . . . , c_(n) _(j)(JCRS_(i)*)=c(CRS_(in) _(j) ^(j)*), there is no JCRS_(j)εD_(j) that cancause a reduction in one of these costs without simultaneously causingan increase an at least one of the others. As said above, the Pareto setof the problem (2), denoted as Pj, contains all the conflict-free jointcandidate resolution strategies for CDNj that fulfil (4).

The conflict resolution process 120 proposes to resolve the conflicts inSLj by means of a JCRS_(j)* selected from the Pareto set, Pj. To thataim, the conflict resolution process 120 must first search forPareto-optimal solutions from which to choose. In other words, theconflict resolution process 120 must build a suitable subset of thePareto set. Once an appropriate number of conflict-free, Pareto-optimaljoint candidate resolution strategies have been found, the conflictresolution process 120 selects the one considered equitable according toaxiomatic bargaining principles. Axiomatic bargaining is a field of gametheory that provides axioms on how to select solutions with certainproperties, such as equity, to a game. In the present context, we canconsider the selection of the equitable JCRSj as a game involving theaircraft in CDNj. It is clear that an equitable solution to the gameshould be Pareto-optimal, JCRS_(j)*, as a strategy that is notPareto-optimal will not be unanimously preferred by all players (it willnot be equitable to some players). However, Pareto-optimality alone isnot sufficient, as some Pareto-optimal solutions may be considered moreequitable than others. For example, some Pareto-optimal JCRS_(j)* mayresult in very high costs (i.e. time variation) for some aircraft andvery low costs for some other aircraft, while other Pareto-optimalJCRS_(j)* may distribute the costs among the aircraft 16 more equitably.Axiomatic bargaining principles will be used to guide the selection ofthe most equitable JCRS_(j)* among those found, with equity in thiscontext reflecting equality in cost distribution.

The selected most-equitable Pareto-optimal strategy is the one proposedto resolve the conflicts in SLj.

The mathematical method adopted to generate Pareto-optimal solutions to(2) is the linear weighting method, which consists of converting themulti-objective optimization problem into a single-objective one wherethe function to be minimized is a linear combination of the costs

C(C R S₁^(j)), …  , C(C R S_(i)^(j)), …  , C(C R S_(n_(j_(j)))^(j)).

The resulting single-objective minimization problem is stated asfollows:

minimise w(JCRS_(j))=w ₁ c ₁(JCRS_(j))+ . . . +w _(i) c _(i)(JCRS_(j))+. . . +w _(n) _(j) c _(n) _(j) (JCRS_(j))=w ₁ c(CRS₁ ^(j))+ . . . +w_(i) c(CRS_(i) ^(j))+ . . . +w _(n) _(j) c(CRS_(n) _(j) ^(j))

subject to JCRS_(j) εD _(j) ,D _(j) ⊂X _(j)  (5)

The factors wi, with iε{1, . . . , n_(j)}, are called weights and areassumed to be positive and normalized so that

${\sum\limits_{i}w_{i}} = 1.$

Given a combination of values for the weights that comply with the aboveconditions, the solution of the resulting single-objective minimizationproblem (5) is a Pareto-optimal solution of the multi-objectiveminimization problem (2).

The problem of searching for an element of the Pareto set of (2) hasbeen recast as a constrained linear programming problem, which consistsof finding the global minimum of a single-objective constrainedminimization problem where the objective function is a linear functionof the costs associated to the individual candidate resolutionstrategies in a joint candidate resolution strategy.

The generation of candidate resolution strategies is at the core of theconflict resolution process 120. As mentioned above, the final aim ofthe conflict resolution process 120 is to find, for each conflictingaircraft 16, a candidate resolution strategy (i.e. an allowable instanceof aircraft intent) whose corresponding predicted trajectory is feasibleand conflict-free and results in an equitable share of the resolutioncosts for the operator. It has been seen that the search for anequitable, conflict-free joint candidate resolution strategy for aconflict dependent network is based on minimizing a function of thecosts associated to the individual candidate resolution strategies inthe joint candidate resolution strategy. Thus, the generation ofcandidate resolution strategies is at the core of the conflictresolution process 120.

The candidate resolution patterns (CRPs) mentioned above areparameterized instructions used as a template to generate differentinstructions of the same type. The amended instructions would result ina new trajectory that could resolve the conflicts in which the aircraft16 is involved. Examples of instructions that will be used to buildsimple candidate resolution patterns are:

Speed reduction: a sequence of instructions that result in a reducedaircraft speed. A speed reduction may be used to create a delay requiredto avoid coming into conflict with a preceding aircraft.

Speed increase: a sequence of instructions that result in an increasedaircraft speed. A speed increase may be used to gain time required toavoid coming into conflict with a following aircraft.

Altitude change: a sequence of instructions that result in an altitudechange.

Direct-to: a sequence of lateral instructions that result in a new RNAVhorizontal track where the aircraft 16 skips waypoints of the originalprocedure (it flies direct to a downstream waypoint). A direct-to may beused to gain time or to avoid an area of conflict (see FIG. 9 a).

Path stretching: a sequence of lateral instructions that result in a newRNAV horizontal track where waypoints are added to the originalprocedure. Path stretching may be used to create a delay or to avoid anarea of conflict (see FIG. 9 b).

When revising aircraft intent data 28 to remove a conflict, the natureof the conflict for the aircraft 16 currently being considered isdetermined. For example, whether the conflict arises because the currentaircraft 16 is catching up with the preceding aircraft 16 may bedetermined. If so, CRPs that create a delay may be selected.Alternatively, if the conflict arises because the current aircraft 16 isfalling behind and coming into conflict with a following aircraft 16,CRPs that give rise to gains in time may be selected. As a furtheralternative, conflicts arising from paths that cross rather thanconverge may see CRPs including an altitude change selected.

Once a CRP is selected, random changes to parameters of the aircraftintent 28 may be made, optionally within limits, to generate thecandidate resolution strategies. For example, random altitude changesmay be used, or random speed changes may be used. The candidateresolution strategies generated in this way for each aircraft 16 may begrouped into joint candidate resolution strategies and the best jointcandidate resolution strategies may be selected, as described above.

Considering the different concepts introduced above, there follows abrief step-by-step description of a full run of the conflict resolutionprocess 120, which is schematically explained by FIG. 10.

1. When a run of the conflict detection process 110 is completed at 701,the conflict detection process 110 calls the conflict resolution process120 at 702. The conflict detection process 110 provides the conflictresolution process 120 with the required conflict-related information,namely conflict dependent networks and conflict sub-lists.

2. The conflict resolution process 120 proceeds one conflict dependentnetwork at a time starting at 703, simultaneously considering all theconflicts in a sub-list.

3. For any network CDNj, the resolution of the conflicts in SLj is basedon a set of joint candidate resolution patterns (JCRPs) for CDNj. AJCRPj is a JCRSj made up of candidate resolution patterns,JCRP_(j)={CRP₁ ^(j), . . . , CRP_(i) ^(j), . . . , CRP_(n) _(j) ^(j)}.To generate a JCRPj at 704, a candidate resolution pattern must beassigned to each of the aircraft in CDNj. In principle, any allowablecandidate resolution pattern for A_(i) ^(j) could be selected as CRP_(i)^(j). The only restriction on the candidate resolution patterns in JCRPjcomes from the fact that, when a conflict involves two aircraft with noearlier conflicts in SLj, at least one of the two aircraft must act uponthe conflict. Consequently, the candidate resolution pattern assigned toat least one of the two aircraft must include an alternative sequence ofinstructions that changes the aircraft intent data prior to theinitiation of the conflict interval (the sequence must be triggeredbefore the conflict starts). A series of heuristics will be in place toguide the selection of allowable candidate resolution patterns for A_(i)^(j) and the definition of the parameters and trigger conditions of thealternative sequences included in the selected candidate resolutionpatterns, as described above. These heuristics will be based on thepreferred intent of A_(i) ^(j) and the attributes of the conflicts inwhich it is involved. For example, the position of the conflict intervalalong the prediction timeline will help determine the triggers of thealternative instructions and the intensity and duration of the conflictswill help define the values of their parameters.

4. At 705, the conflict detection process 110 is called by the conflictresolution process 120 to check whether the generated JCRPjs areconflict-free. If no conflict-free JCRPjs can be found at 706, heuristicmethods are employed at 707 to extend CDNj by including the aircraft 16interfering with the JCRPjs. Thus, it is implicitly assumed that thereason why the allowable joint conflict resolution patterns do notresult in a conflict-free conflict dependent network is because theycreate conflicts with aircraft 16 outside the network. If an interferingaircraft 16 is itself including in a conflict dependent network, thenthat conflict dependent network must be considered in combination withCDNj for conflict resolution.

5. The resulting conflict-free JCRPjs are considered as the initialJCRSjs to initiate the search for Pareto-optimal conflict-freeJCRS*_(j)s at 708.

6. A subset of the Pareto set, i.e. set of conflict-free JCRS*_(j)s isbuilt at 709. To generate this subset, the minimization problem in (6)must be repeatedly solved for different sets of values for the weights,so as to obtain Pareto-optimal solutions that cover all areas of thePareto set. To resolve the minimization problem, a stochasticoptimization algorithm is employed. This algorithms will search for theminimum of w(JCRSj) from among JCRSjs generated from the initial jointconflict resolution patterns by randomizing the parameters and triggerconditions of the alternative instructions introduced in the CRP_(i)^(j)s.

7. Once a set of conflict-free Pareto-optimal solutions JCRS*_(j)s isavailable, the most equitable solution among the ones obtained isselected at 710 as the Joint Resolution Strategy for CDNj, denoted asJRSj.

8. Steps 3 to 7 are performed for each of the identified conflictdependent networks. The joint resolution strategy for all theconflicting aircraft is the combination of the JRSjs obtained for thedifferent CDNjs

Variations

It will be clear to the skilled person that modifications may be made tothe embodiments described above without departing from the scope of thedisclosure.

For example, the present disclosure enjoys particular benefit whenapplied to air traffic management dealing with the most challengingscenario of predominantly converging paths such as terminal arrivals.Nonetheless, the present disclosure will of course also bring benefitsto less challenging environments like diverging paths as for terminaldepartures and also crossing paths.

It will be appreciated that the location of parts of the presentdisclosure may be varied. For example, trajectories may be calculated byground-based or air-based systems. For example, the air trafficmanagement may be ground-based, but need not necessarily be so. The airtraffic management need not be centralized. For example, a distributedair-based system could be possible. Different air traffic management maycooperate and share information. For example, air traffic managementhaving responsibility for adjacent airspaces may pass trajectoryinformation for aircraft anticipated to cross between the adjacentairspaces.

1. A computer-implemented method of managing airspace through which aplurality of aircraft are flying, comprising: receiving, from theaircraft, user preferred aircraft intent data that unambiguously definesthe user preferred trajectory of each aircraft; calling an initialconflict detection procedure comprising: calculating the correspondinguser preferred trajectories from the user preferred aircraft intentdata; and comparing the user preferred trajectories so as to identifyone or more conflicts between trajectories and to identify conflictedaircraft predicted to fly the identified conflicting trajectories;calling an initial conflict resolution procedure comprising: revisingthe user preferred aircraft intent data of one or more of the conflictedaircraft to produce revised aircraft intent data that will unambiguouslydefine a corresponding revised trajectory; and sending the revisedaircraft intent data to the corresponding conflicted aircraft.
 2. Themethod of claim 1 further comprising, after calling the initial conflictresolution procedure, calling a further conflict detection procedure,wherein the further conflict detection comprises: calculating revisedtrajectories from the corresponding revised aircraft intent data;comparing the user-preferred trajectories of aircraft not subject torevised aircraft intent data and revised trajectories from the aircraftsubject to revised aircraft intent data so as to identify one or moreconflicts between trajectories, and to identify still-conflictedaircraft predicted to fly the identified conflicting trajectories; andif conflicts are identified during the further conflict detectionprocedure, either calling a further conflict resolution procedure orindicating that no further conflict resolution processing will takeplace, wherein the further conflict resolution procedure comprises:revising the user preferred aircraft intent data and/or revised aircraftintent data of one or more of the still-conflicted aircraft, and callingthe further conflict detection procedure; or if no conflicts areidentified during the further conflict detection procedure, continuingto the step of sending the revised aircraft intent data to thecorresponding conflicted aircraft.
 3. The method of claim 2, furthercomprising receiving from an aircraft an indication that revisedaircraft intent data proposed for that aircraft is rejected, performinga rejection procedure comprising: calling the further conflictresolution procedure in which the rejected revised aircraft intent datais further revised and, if it is indicated that no further conflictprocessing will take place, advising the aircraft that rejected therevised aircraft intent data that that aircraft must accept the rejectedrevised aircraft intent data.
 4. The method of claim 3, comprisingperforming the rejection procedure by locking aircraft whoseuser-preferred aircraft intent data was not revised and aircraft whoaccepted revised aircraft intent data, the remaining aircraft beingunlocked aircraft with rejected revised aircraft intent data and, duringfurther conflict resolution procedures called by the rejectionprocedure, not revising aircraft intent data of locked aircraft andrevising aircraft intent data of one or more unlocked aircraft.
 5. Themethod of claim 1, wherein the step of comparing the user preferredtrajectories so as to identify one or more conflicts betweentrajectories, during the initial conflict detection procedure and/orduring the further conflict detection procedure, comprises forming pairsbetween each of the conflicted aircraft and, for each pair, comparingthe separation of the aircraft on their trajectories at discrete timepoints as their trajectories progress.
 6. The method of claim 4,comprising applying heuristics to identify pairs of aircraft that cannotcome into conflict at a particular discrete time point.
 7. The method ofclaim 6, comprising using the positions of aircraft in a pair todetermine that they cannot come into conflict for certain number ofdiscrete time points.
 8. The method of claim 1, wherein the initialconflict detection procedure is repeated according to any one or moreof: predetermined intervals, periodically, whenever an aircraft entersthe airspace, and when new user-preferred aircraft intent data isreceived from an aircraft.
 9. A method of operating an aircraft within amanaged airspace, comprising: the aircraft sending user preferredaircraft intent data that unambiguously defines the user preferredtrajectory of the aircraft to an air traffic management facility; theaircraft receiving from the air traffic management facility, revisedaircraft intent data that unambiguously defines a corresponding revisedtrajectory of the aircraft; the aircraft calculating and displaying therevised trajectory; and the aircraft following the revised trajectory.10. The method of claim 9, further comprising: after displaying therevised trajectory, the aircraft sending to the air traffic managementfacility an indication that the revised aircraft intent data isrejected; the aircraft receiving from the air traffic managementfacility further revised aircraft intent data or an indication that therevised aircraft intent data must be accepted; and the aircraftfollowing either the further revised aircraft intent data or the revisedaircraft intent data, as appropriate. 11-16. (canceled)
 17. An airtraffic control apparatus arranged to manage airspace through which aplurality of aircraft are flying, comprising: communication meansconfigured for bi-directional communication with the aircraft; andprocessing means in operative communication with the communication meansso as to allow transfer of data to and from the aircraft; wherein theprocessing means comprises: a traffic management component, a trajectorycomputation component and data storage means accessible to both thetraffic management component and the trajectory computation component;wherein the processing means is configured to receive, from the aircraftand via the communication means, user-preferred aircraft intent datadescribing unambiguously the aircraft's user-preferred trajectory ofeach aircraft and to store the user-preferred aircraft intent data inthe data storage means; the traffic management component is configuredto call an initial conflict detection procedure comprising: the trafficmanagement component calling the trajectory computation component tocalculate the corresponding user preferred trajectories from the userpreferred aircraft intent data; and the traffic management componentcomparing the user preferred trajectories so as to identify one or moreconflicts between trajectories and to identify conflicted aircraftpredicted to fly the identified conflicting trajectories; the trafficmanagement component is configured to call an initial conflictresolution procedure comprising: the traffic management componentrevising the user preferred aircraft intent data of one or more of theconflicted aircraft to produce revised aircraft intent data that willunambiguously define a corresponding revised trajectory; the trafficmanagement component is configured to perform a further conflictdetection procedure comprising: the traffic management component callingthe trajectory computation component to calculate the correspondingrevised trajectories from the revised aircraft intent data; and thetraffic management component comparing the user-preferred trajectoriesfrom aircraft not subject to revised aircraft intent data and revisedtrajectories from aircraft subject to revised trajectories so as toidentify one or more conflicts between trajectories and to identifystill-conflicted aircraft predicted to fly the identified conflictingtrajectories; the traffic management component is configured: ifconflicts are identified during the further conflict detectionprocedure, either to call a further conflict resolution procedure or toindicate that no further conflict resolution processing will take place,wherein the further conflict detection procedure comprises revising theuser preferred aircraft intent data and/or revised aircraft intent dataof one or more of the still-conflicted aircraft and to call the furtherconflict detection procedure, or if no conflicts are identified duringthe further conflict detection procedure, to continue to a step oftransmitting revised aircraft intent data; and the processing means isconfigured, when no conflicts remain in the trajectories, to pass to thecommunication means the revised aircraft intent data and thecommunication means is configured to perform the step of transmittingrevised aircraft intent data by transmitting the revised aircraft intentdata to the associated aircraft.
 18. An aircraft comprising: airbornecommunication means for bidirectional communication with an air trafficcontrol apparatus, and airborne processing means in operativecommunication with the airborne communication means so as to allowtransfer of data to and from the air traffic control apparatus; whereinthe airborne processing means comprises a flight management component, atrajectory computation component and data storage means accessible toboth the flight management component and the airborne trajectorycomputation component; and wherein: the flight management component isconfigured to provide user-preferred aircraft intent data that describesunambiguously the aircraft's user-preferred trajectory; the airborneprocessing means is configured to store in the data storage means theuser-preferred aircraft intent data provided by the flight managementcomponent; the airborne processing means is configured to transmit theuser-preferred aircraft intent data to the air traffic control apparatusvia the airborne communication means; the airborne processing means isconfigured to receive from the air traffic control apparatus, revisedaircraft intent data that unambiguously defines a corresponding revisedtrajectory of the aircraft, and to store in the data storage means therevised aircraft intent data; the airborne trajectory computationcomponent is configured to calculate the revised trajectory from therevised aircraft intent data stored in the data storage means; theairborne processing means is configured to display a representation ofthe trajectory; and the airborne processing means is configured totransmit an indication that the revised aircraft intent data is rejectedto the air traffic control apparatus via the airborne communicationmeans; the airborne processing means is configured to receive from theair traffic control apparatus, further revised aircraft intent data oran indication that the revised aircraft intent data must be accepted;and the aircraft is configured to follow either the further revisedaircraft intent data or the revised aircraft intent data.
 19. A systemfor managing airspace through which a plurality of aircraft are flying,the system comprising: a computer apparatus; a non-transitory computerreadable medium comprising instructions stored thereon, that whenexecuted by the computer apparatus, causes the computer apparatus to:receive, from the aircraft, user preferred aircraft intent data thatunambiguously defines the user preferred trajectory of each aircraft;call an initial conflict detection procedure comprising: calculating thecorresponding user preferred trajectories from the user preferredaircraft intent data; and comparing the user preferred trajectories soas to identify one or more conflicts between trajectories and toidentify conflicted aircraft predicted to fly the identified conflictingtrajectories; call an initial conflict resolution procedure comprising:revising the user preferred aircraft intent data of one or more of theconflicted aircraft to produce revised aircraft intent data that willunambiguously define a corresponding revised trajectory; and send therevised aircraft intent data to the corresponding conflicted aircraft.20. The system of claim 19 wherein said non-transitory computer readablemedium comprises further instructions, that when executed by thecomputer apparatus, causes the computer apparatus to, after calling theinitial conflict resolution procedure, call a further conflict detectionprocedure, wherein the further conflict detection comprises: calculatingrevised trajectories from the corresponding revised aircraft intentdata; comparing the user-preferred trajectories of aircraft not subjectto revised aircraft intent data and revised trajectories from theaircraft subject to revised aircraft intent data so as to identify oneor more conflicts between trajectories, and to identify still-conflictedaircraft predicted to fly the identified conflicting trajectories; andif conflicts are identified during the further conflict detectionprocedure, either calling a further conflict resolution procedure orindicating that no further conflict resolution processing will takeplace, wherein the further conflict resolution procedure comprises:revising the user preferred aircraft intent data and/or revised aircraftintent data of one or more of the still-conflicted aircraft, and callingthe further conflict detection procedure; or if no conflicts areidentified during the further conflict detection procedure, continuingto the step of sending the revised aircraft intent data to thecorresponding conflicted aircraft.
 21. The system of claim 20 whereinsaid non-transitory computer readable medium comprises furtherinstructions, that when executed by the computer apparatus, causes thecomputer apparatus to: receive from an aircraft an indication thatrevised aircraft intent data proposed for that aircraft is rejected; andperform a rejection procedure comprising: calling the further conflictresolution procedure in which the rejected revised aircraft intent datais further revised and, if it is indicated that no further conflictprocessing will take place, advising the aircraft that rejected therevised aircraft intent data that that aircraft must accept the rejectedrevised aircraft intent data.
 22. The system of claim 21, whereininstructions for performing the rejection procedure compriseinstructions to: lock aircraft whose user-preferred aircraft intent datawas not revised and aircraft who accepted revised aircraft intent data,the remaining aircraft being unlocked aircraft with rejected revisedaircraft intent data; and during further conflict resolution procedurescalled by the rejection procedure, not revise aircraft intent data oflocked aircraft and revise aircraft intent data of one or more unlockedaircraft.
 23. The system of claim 20, wherein instructions for comparingthe trajectories so as to identify one or more conflicts betweentrajectories, during the initial conflict detection procedure and/orduring the further conflict detection procedure, comprises instructionsto: form pairs between each of the conflicted aircraft; and for eachpair, compare the separation of the aircraft on their trajectories atdiscrete time points as their trajectories progress.
 24. The system ofclaim 22 wherein said non-transitory computer readable medium comprisesfurther instructions, that when executed by the computer apparatus,causes the computer apparatus to applying heuristics to identify pairsof aircraft that cannot come into conflict at a particular discrete timepoint.
 25. The system of claim 24 wherein said non-transitory computerreadable medium comprises further instructions, that when executed bythe computer apparatus, causes the computer apparatus to use thepositions of aircraft in a pair to determine that they cannot come intoconflict for certain number of discrete time points.
 26. Anon-transitory computer readable medium having stored thereon a computerprogram for managing airspace through which a plurality of aircraft areflying, the computer program comprising instructions that when executedby a computer apparatus causes the computer apparatus to: receive, fromthe aircraft, user preferred aircraft intent data that unambiguouslydefines the user preferred trajectory of each aircraft; call an initialconflict detection procedure comprising: calculating the correspondinguser preferred trajectories from the user preferred aircraft intentdata; and comparing the user preferred trajectories so as to identifyone or more conflicts between trajectories and to identify conflictedaircraft predicted to fly the identified conflicting trajectories; callan initial conflict resolution procedure comprising: revising the userpreferred aircraft intent data of one or more of the conflicted aircraftto produce revised aircraft intent data that will unambiguously define acorresponding revised trajectory; and send the revised aircraft intentdata to the corresponding conflicted aircraft.